Flow ducts for flue gas can include flow-distributing elements or baffles, especially in the vicinity of an elbow or a bend having flow guide surfaces.
These flow ducts for the flue gas of an flue gas cleaning plant can be located downstream of a boiler furnace, especially a power plant.
The flow ducts are usually made of sheet metal and have primarily a rectangular or a square flow duct cross section.
In this description of our invention, the term "flow duct" and similar terms not only mean a conducting pipe, but also flow equipment and equipment parts, especially admission equipment for the gas to be cleaned and discharge equipment for the cleaned gas from the wash tower for the wet flue gas cleaning.
In the wet flue gas cleaning of the flue gas downstream of the boiler furnace, special requirements characterize the flow guide elements. On the one hand, the flow guide elements have increased requirements in regard to mechanical considerations related to resistance to creep, also to thermal stress due to temperature changes. On the other hand, the flow guide elements must significantly resist chemical and electro-chemical corrosion which occurs because the flue gas to be cleaned, also that which has been cleaned, travel with corrosive components.
Abrasive action occurs because the above-named flue gas entrains fine solids, which also tend to deposit on the flow guide surfaces which again can lead to formation of corrosion-promoting microelements.
Finally, the flow resistance of the flow guide elements must be sufficiently small to avoid unnecessary energy losses.
In the known flow duct of the above-described structure and purpose, the flow guide elements are made of sheet metal. Because of the corrosive action, high grade austenitic chromium sheet metal or chromium/nickel sheet metal is used. The operational lifetime is nonetheless unsatisfactory. It is frequently under 2000 operating hours. The surface roughness is comparatively large. Because of the relatively high surface roughness, the flow resistance is disturbingly large, and the rough surface promotes the deposition of the solid components from the gas.
Furthermore, an electric potential acting corrosively is established between the flow guide elements of the comparatively noble alloy components and the metallic material of the flow duct which primarily is made of structural steel.
To improve the operational life of the duct, it is known to coat the flow guide elements with a protective layer made of polytetrafluoroethylene, for example, or some other material. However, experience has shown that those expedients do not lead to noteworthy improvement of the operational lifetime of the flow guide elements.